Optical networks are originally predestined for transmission of data traffic at high bit rates. There are a number of transmission standards which were specifically developed for high speed transmission of optical signals in modern optical networks.
Synchronous Optical NETwork (SONET) and Synchronous Digital Hierarchy (SDH) describe two families of closely related and compatible standards that govern interface parameters; rates, formats and multiplexing methods; operations, administration, maintenance and provisioning for high-speed signal transmission. SONET is primarily a set of North American standards with a fundamental transport rate beginning at approximately 52 Mb/s (i.e., 51.84 Mb/s), while SDH, principally used in Europe and Asia, defines a basic rate near 155 Mb/s (to be precise, 51.84×3=155.52 Mb/s). From a transmission perspective, together they provide an international basis for supporting both existing and new services in the developed and developing countries.
For transmitting data, SDH and SONET use frame formats transmitted every 125 μs (8000 frames/sec). Because of compatibility between SDH and SONET, their basic frames are similarly structured, but differ in dimension which fact reflects the basic transmission rates of 155.52 and 51.84 Mb/s, respectively. To be more specific, a basic frame format of SDH is 9 rows of 270 bytes, or 2430 bits/frame, corresponding to an aggregate frame rate of 155.52 Mb/s. For SDH systems, the mentioned basic frame transmitted at the rate 155.52 Mb/s forms the fundamental building block called Synchronous Transport Module Level-1 (STM-1). For SONET systems, the basic frame has dimensions of 9 rows by 90 byte columns and, being transmitted at the rate 51.84 Mb/s, forms the appropriate fundamental building block called Synchronous Transport Signal Level-1 (STS-1).
Lower rate payloads (data portions transmitted at rates smaller than the basic ones) are mapped into the fundamental building blocks, while higher rate signals are generated by synchronously multiplexing N fundamental building blocks to form STM-N signals (in SDH) or STS-N signals (in SONET).
Each basic frame in SONET or SDH comprises an information portion called Information Payload and a service portion called Overhead (OH), the latter being subdivided into a number of areas of overhead bytes (for example, Path Overhead—POH, Transport Overhead—TOH) predestined for various service and control functions. One of such areas is a column of Path Overhead (POH) usually residing within the Information Payload area and comprising a plurality of bytes. POH supports performance monitoring, status feedback, signal labeling, user channel and a tracing function in a path. This overhead is added and dismantled at or near the service origination/termination points defining the path, and is not processed at intermediary nodes.
One of the important bytes of the POH is a Path Trace byte called J1. This byte is used to transmit repetitively a Path Access Point Identifier so that a path receiving terminal can verify its continued connection to the intended transmitter. In transport networks operating according to SONET, J1 is used to send a repetitive signal to form a 64 byte string (trace), while in networks utilizing SDH the repetitive signal produced by J1 byte is preferably in a format of 16 byte string (trace). However, in the SDH standard there is an option of a 64 byte free format string, and where the 16 byte format is transferred in the 64 byte field, it shall be repeated four times. The path terminating equipment (PTE), depending on the standard in use, must therefore be able to continuously compare either a 16 byte long string, or a 64 byte long string with an expected code of the J1 string (trace).
The SDH multiplexing structure, defined in ITU-T Recommendation G.707(03/96), comprises so-called virtual containers serving to combine lower rate payloads such as by mapping into these containers and adding POH. The combined payloads fitted with POH are further aligned and multiplexed in order to form an STM-N signal. The STM-N signal can be obtained either by multiplexing AU-3 signal (accepted also in SONET) by 3N, or by multiplexing N signals AU-4. AU-4 is formed by adding pointers to a VC-4 signal (virtual container level-4). Similarly, the AU-3 signal is formed from VC-3 by adding AU-3 pointers. Lower level signals TU-11, TU-12, TU-2 and TU-3, which are formed by adding respective pointers to lower level virtual containers VC-11, VC-12, VC-2 and VC-3, in their turn serve as components for composing the higher level virtual containers VC-3 or VC-4. Packet networks such as IP, Ethernet, ATM, FC (Fiber channel) operate according to protocols defining burst-like transmission of information units called packets or cells, wherein the length and contents of the packets in each specific network are predetermined by the suitable standards and protocols.
Presently, not only the modern LANs grew, but as a rule, they are now interconnected by wide area networks (WANs) which operate according to totally different protocols. That is a result of one of the trends in the modern world of communications where integration of various types of networks becomes more and more popular. For example, a communication path between two end users or providers may include both network sections utilizing packet framing (such as IP or Ethernet), and sections of optical networks such as SDH or SONET utilizing so-called virtual containers which are complex aggregated structures of digital frames.
For transmitting Ethernet packets, digital frames of SONET/SDH envelope the information comprised in the Ethernet packets (SONET/SDH is generally considered a 1st (lower) level, and Ethernet—a 2nd (higher) level.
Protection of data traffic in optical rings has been developed and standardized for the above-mentioned two accepted standards—SDH and SONET.
For optical networks transmitting SDH traffic, there exists a so-called MS-SPRING (Multiplex Section Shared Protection Ring) system of traffic protection described in an ITU-T Standard Recommendation G.841.
For the SONET optical networks, a so-called BLSR (Bi-Directional Line Switched Ring) protection system was proposed, defined in the North American Standard of Bellcor GR.1230.
The MS-SPRING system proposes two options for protecting traffic in optical rings. The first option is developed for optical rings formed by two-fiber links connecting network elements, wherein each fiber serves one of two opposite traffic directions in the ring. Half the capacity of each fiber is intended for the protection purposes. The second option is intended for rings with four-fiber links, where each direction of optical traffic between the network elements is served by two optical fibers. One of the two fibers of one and the same direction is intended for the purposes of protection.
The MS-SPRING system completely performs its functions for SDH traffic when a fiber cut occurs in the ring and the traffic should be redirected. Ethernet traffic, which is a layer 2 data traffic, can be transmitted in an optical network over SDH traffic. Usually, for carrying the Ethernet data, one may use one or more AU-4 containers forming the SDH data stream. It should be noted, that some AU-4 containers may be free of carrying the Ethernet data. In SONET networks, AU-3 containers are used for transporting the Ethernet information.
The MS-SPRING successfully copes with protecting both SDH traffic and the Ethernet traffic over SDH in the optical ring in case of a fiber cut between two adjacent network elements. In such a case each of the two network elements redirects the traffic, which should have been sent via the cut fiber, to the opposite direction and thereby the traffic forms another ring contour using the protective capacity of the fiber (fibers) assigned to the opposite direction. A case of such a fault in a two-fiber ring network is illustrated in FIG. 1a. It is assumed that each node N1 to N6 of network 10 is ADM (Add Drop Multiplexer), i.e. a network element providing for dropping some data channels to a customer (not shown) connected to the node, as well as for adding some data channels from the customer, to be transmitted to other nodes in the ring. In the case of a single fiber cut (shown as a cross) functions of the nodes are not affected.
There is another case of fault in a the ring network, called “an isolated node” that happens as a result of either a node failure, or a double fiber cut where the two fiber links surrounding one node are cut. The case of isolated node in a two-fiber ring network 20 is shown in FIG. 1b. In such a case, the MS-SPRING system, though coping with protecting the SDH traffic, is ineffective in protecting the Ethernet data.
There are two mutually interconnected reasons for that.
The first reason is that when an isolated node appears, the MS-SPRING protocol initiates a so-called squelching algorithm, according to which any traffic which originates or terminates at the isolated node should be squelched by other nodes. The traffic which is affected is the SDH virtual containers.The second reason is that the Ethernet layer traffic enveloped in the SDH virtual container(s) must perform termination/generation operations at every node of the ring, regardless whether a particular Ethernet packet in such a container is addressed to this specific node or goes through to another node. Indeed, if one node fails, the MS-SPRING system will automatically consider all AU-4 SDH containers carrying the Ethernet traffic as terminating/originating at the faulty node, and thereby will mark them to be squelched.
Due to these two reasons, AU-4 virtual containers of SDH traffic comprising the Ethernet packets (and possibly some AU-4s which do not comprise Ethernet data but are to be added/dropped at the faulty node) will be squelched as those terminated/generated at the isolated node. It means that the task of protecting Ethernet traffic cannot be fulfilled since actually all the Ethernet data will be lost.
Yet another kind of faults is known—a so-called “isolated section”—when two (or more) fiber cuts occur in a ring network and separate from it more than one nodes. An example of such a fault is illustrated in FIG. 1c. For each part of the network between two fiber cuts, all nodes belonging to the other separated part(s) of the network constitute “isolated nodes”, with all the consequences described above.
MS-SPRING in its standard version is unsuitable for protecting Ethernet traffic in the cases of occurring one or more isolated nodes in a ring, and one of solutions to protect it is to use STP (Spanning Tree Protocol) in addition to MS-SPRING. The STP protocol is a complex, high volume software tool which is capable of routing traffic in any network topology in the presence of any cases of faults. For ring networks, the STP protocol is too heavy, expensive and slow.